Re: [PATCH v2 5/6] Add use of RCU for qemu_logfile.

[Alex Bennée]

Robert Foley <address@hidden> writes:

> On Mon, 18 Nov 2019 at 07:22, Alex Bennée <address@hidden> wrote:
>>
>>
>> > +    if (logfile) {
>> > +        qemu_flockfile(logfile->fd);
>> > +        return logfile->fd;
>> > +    } else {
>> > +        rcu_read_unlock();
>>
>> As qemu_log_lock() and unlock should be paired we can drop the unlock
>> here and do an unconditional unlock bellow even if a null fd is passed.
>>
>> Otherwise:
>>
>> Reviewed-by: Alex Bennée <address@hidden>
>>
> Sounds reasonable.  It sounds like we should change it to be something
> like this.
>
> static inline FILE *qemu_log_lock(void)
> {
>     QemuLogFile *logfile;
>     rcu_read_lock();
>     logfile = atomic_rcu_read(&qemu_logfile);
>     if (logfile) {
>         qemu_flockfile(logfile->fd);
>         return logfile->fd;
>     }
>     rcu_read_unlock();
>     return NULL;
> }
> I will make the changes.

No I mean as you have to call qemu_log_unlock() you can to the unlock
there. You have to hold the lock over the usage of the resource:

  static inline FILE *qemu_log_lock(void)
  {
      QemuLogFile *logfile;
      rcu_read_lock();
      logfile = atomic_rcu_read(&qemu_logfile);
      if (logfile) {
          qemu_flockfile(logfile->fd);
          return logfile->fd;
      } else {
          return NULL;
      }
  }

  static inline void qemu_log_unlock(FILE *fd)
  {
      if (fd) {
          qemu_funlockfile(fd);
      }
      rcu_read_unlock();
  }


>
> Thanks,
> -Rob
> On Mon, 18 Nov 2019 at 07:22, Alex Bennée <address@hidden> wrote:
>>
>>
>> Robert Foley <address@hidden> writes:
>>
>> > This now allows changing the logfile while logging is active,
>> > and also solves the issue of a seg fault while changing the logfile.
>> >
>> > Any read access to the qemu_logfile handle will use
>> > the rcu_read_lock()/unlock() around the use of the handle.
>> > To fetch the handle we will use atomic_rcu_read().
>> > We also in many cases do a check for validity of the
>> > logfile handle before using it to deal with the case where the
>> > file is closed and set to NULL.
>> >
>> > The cases where we write to the qemu_logfile will use atomic_rcu_set().
>> > Writers will also use call_rcu() with a newly added qemu_logfile_free
>> > function for freeing/closing when readers have finished.
>> >
>> > Signed-off-by: Robert Foley <address@hidden>
>> > ---
>> > v2
>> >     - No specific changes, just merging in cleanup changes in 
>> > qemu_set_log().
>> > ---
>> > v1
>> >     - Changes for review comments.
>> >     - Minor changes to definition of QemuLogFile.
>> >     - Changed qemu_log_separate() to fix unbalanced and
>> >       remove qemu_log_enabled() check.
>> >     - changed qemu_log_lock() to include else.
>> >     - make qemu_logfile_free static.
>> >     - use g_assert(logfile) in qemu_logfile_free.
>> >     - Relocated unlock out of if/else in qemu_log_close(), and
>> >       in qemu_set_log().
>> > ---
>> >  include/qemu/log.h | 42 +++++++++++++++++++++++----
>> >  util/log.c         | 72 ++++++++++++++++++++++++++++++++--------------
>> >  include/exec/log.h | 33 ++++++++++++++++++---
>> >  tcg/tcg.c          | 12 ++++++--
>> >  4 files changed, 126 insertions(+), 33 deletions(-)
>> >
>> > diff --git a/include/qemu/log.h b/include/qemu/log.h
>> > index a7c5b01571..528e1f9dd7 100644
>> > --- a/include/qemu/log.h
>> > +++ b/include/qemu/log.h
>> > @@ -3,9 +3,16 @@
>> >
>> >  /* A small part of this API is split into its own header */
>> >  #include "qemu/log-for-trace.h"
>> > +#include "qemu/rcu.h"
>> > +
>> > +typedef struct QemuLogFile {
>> > +    struct rcu_head rcu;
>> > +    FILE *fd;
>> > +} QemuLogFile;
>> >
>> >  /* Private global variable, don't use */
>> > -extern FILE *qemu_logfile;
>> > +extern QemuLogFile *qemu_logfile;
>> > +
>> >
>> >  /*
>> >   * The new API:
>> > @@ -25,7 +32,16 @@ static inline bool qemu_log_enabled(void)
>> >   */
>> >  static inline bool qemu_log_separate(void)
>> >  {
>> > -    return qemu_logfile != NULL && qemu_logfile != stderr;
>> > +    QemuLogFile *logfile;
>> > +    bool res = false;
>> > +
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile && logfile->fd != stderr) {
>> > +        res = true;
>> > +    }
>> > +    rcu_read_unlock();
>> > +    return res;
>> >  }
>> >
>> >  #define CPU_LOG_TB_OUT_ASM (1 << 0)
>> > @@ -55,14 +71,23 @@ static inline bool qemu_log_separate(void)
>> >
>> >  static inline FILE *qemu_log_lock(void)
>> >  {
>> > -    qemu_flockfile(qemu_logfile);
>> > -    return logfile->fd;
>> > +    QemuLogFile *logfile;
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        qemu_flockfile(logfile->fd);
>> > +        return logfile->fd;
>> > +    } else {
>> > +        rcu_read_unlock();
>>
>> As qemu_log_lock() and unlock should be paired we can drop the unlock
>> here and do an unconditional unlock bellow even if a null fd is passed.
>>
>> Otherwise:
>>
>> Reviewed-by: Alex Bennée <address@hidden>
>>
>> > +        return NULL;
>> > +    }
>> >  }
>> >
>> >  static inline void qemu_log_unlock(FILE *fd)
>> >  {
>> >      if (fd) {
>> >          qemu_funlockfile(fd);
>> > +        rcu_read_unlock();
>> >      }
>> >  }
>> >
>> > @@ -73,9 +98,14 @@ static inline void qemu_log_unlock(FILE *fd)
>> >  static inline void GCC_FMT_ATTR(1, 0)
>> >  qemu_log_vprintf(const char *fmt, va_list va)
>> >  {
>> > -    if (qemu_logfile) {
>> > -        vfprintf(qemu_logfile, fmt, va);
>> > +    QemuLogFile *logfile;
>> > +
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        vfprintf(logfile->fd, fmt, va);
>> >      }
>> > +    rcu_read_unlock();
>> >  }
>> >
>> >  /* log only if a bit is set on the current loglevel mask:
>> > diff --git a/util/log.c b/util/log.c
>> > index 91ebb5c924..9f9b6b74b7 100644
>> > --- a/util/log.c
>> > +++ b/util/log.c
>> > @@ -28,7 +28,7 @@
>> >
>> >  static char *logfilename;
>> >  static QemuMutex qemu_logfile_mutex;
>> > -FILE *qemu_logfile;
>> > +QemuLogFile *qemu_logfile;
>> >  int qemu_loglevel;
>> >  static int log_append = 0;
>> >  static GArray *debug_regions;
>> > @@ -37,10 +37,14 @@ static GArray *debug_regions;
>> >  int qemu_log(const char *fmt, ...)
>> >  {
>> >      int ret = 0;
>> > -    if (qemu_logfile) {
>> > +    QemuLogFile *logfile;
>> > +
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> >          va_list ap;
>> >          va_start(ap, fmt);
>> > -        ret = vfprintf(qemu_logfile, fmt, ap);
>> > +        ret = vfprintf(logfile->fd, fmt, ap);
>> >          va_end(ap);
>> >
>> >          /* Don't pass back error results.  */
>> > @@ -48,6 +52,7 @@ int qemu_log(const char *fmt, ...)
>> >              ret = 0;
>> >          }
>> >      }
>> > +    rcu_read_unlock();
>> >      return ret;
>> >  }
>> >
>> > @@ -56,12 +61,24 @@ static void __attribute__((__constructor__)) 
>> > qemu_logfile_init(void)
>> >      qemu_mutex_init(&qemu_logfile_mutex);
>> >  }
>> >
>> > +static void qemu_logfile_free(QemuLogFile *logfile)
>> > +{
>> > +    g_assert(logfile);
>> > +
>> > +    if (logfile->fd != stderr) {
>> > +        fclose(logfile->fd);
>> > +    }
>> > +    g_free(logfile);
>> > +}
>> > +
>> >  static bool log_uses_own_buffers;
>> >
>> >  /* enable or disable low levels log */
>> >  void qemu_set_log(int log_flags)
>> >  {
>> >      bool need_to_open_file = false;
>> > +    QemuLogFile *logfile;
>> > +
>> >      qemu_loglevel = log_flags;
>> >  #ifdef CONFIG_TRACE_LOG
>> >      qemu_loglevel |= LOG_TRACE;
>> > @@ -80,43 +97,47 @@ void qemu_set_log(int log_flags)
>> >      g_assert(qemu_logfile_mutex.initialized);
>> >      qemu_mutex_lock(&qemu_logfile_mutex);
>> >      if (qemu_logfile && !need_to_open_file) {
>> > -        qemu_mutex_unlock(&qemu_logfile_mutex);
>> > -        qemu_log_close();
>> > +        logfile = qemu_logfile;
>> > +        atomic_rcu_set(&qemu_logfile, NULL);
>> > +        call_rcu(logfile, qemu_logfile_free, rcu);
>> >      } else if (!qemu_logfile && need_to_open_file) {
>> > +        logfile = g_new0(QemuLogFile, 1);
>> >          if (logfilename) {
>> > -            qemu_logfile = fopen(logfilename, log_append ? "a" : "w");
>> > -            if (!qemu_logfile) {
>> > +            logfile->fd = fopen(logfilename, log_append ? "a" : "w");
>> > +            if (!logfile->fd) {
>> > +                g_free(logfile);
>> >                  perror(logfilename);
>> >                  _exit(1);
>> >              }
>> >              /* In case we are a daemon redirect stderr to logfile */
>> >              if (is_daemonized()) {
>> > -                dup2(fileno(qemu_logfile), STDERR_FILENO);
>> > -                fclose(qemu_logfile);
>> > +                dup2(fileno(logfile->fd), STDERR_FILENO);
>> > +                fclose(logfile->fd);
>> >                  /* This will skip closing logfile in qemu_log_close() */
>> > -                qemu_logfile = stderr;
>> > +                logfile->fd = stderr;
>> >              }
>> >          } else {
>> >              /* Default to stderr if no log file specified */
>> >              assert(!is_daemonized());
>> > -            qemu_logfile = stderr;
>> > +            logfile->fd = stderr;
>> >          }
>> >          /* must avoid mmap() usage of glibc by setting a buffer "by hand" 
>> > */
>> >          if (log_uses_own_buffers) {
>> >              static char logfile_buf[4096];
>> >
>> > -            setvbuf(qemu_logfile, logfile_buf, _IOLBF, 
>> > sizeof(logfile_buf));
>> > +            setvbuf(logfile->fd, logfile_buf, _IOLBF, 
>> > sizeof(logfile_buf));
>> >          } else {
>> >  #if defined(_WIN32)
>> >              /* Win32 doesn't support line-buffering, so use unbuffered 
>> > output. */
>> > -            setvbuf(qemu_logfile, NULL, _IONBF, 0);
>> > +            setvbuf(logfile->fd, NULL, _IONBF, 0);
>> >  #else
>> > -            setvbuf(qemu_logfile, NULL, _IOLBF, 0);
>> > +            setvbuf(logfile->fd, NULL, _IOLBF, 0);
>> >  #endif
>> >              log_append = 1;
>> >          }
>> > -        qemu_mutex_unlock(&qemu_logfile_mutex);
>> > +        atomic_rcu_set(&qemu_logfile, logfile);
>> >      }
>> > +    qemu_mutex_unlock(&qemu_logfile_mutex);
>> >  }
>> >
>> >  void qemu_log_needs_buffers(void)
>> > @@ -245,19 +266,28 @@ out:
>> >  /* fflush() the log file */
>> >  void qemu_log_flush(void)
>> >  {
>> > -    fflush(qemu_logfile);
>> > +    QemuLogFile *logfile;
>> > +
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        fflush(logfile->fd);
>> > +    }
>> > +    rcu_read_unlock();
>> >  }
>> >
>> >  /* Close the log file */
>> >  void qemu_log_close(void)
>> >  {
>> > +    QemuLogFile *logfile;
>> > +
>> >      g_assert(qemu_logfile_mutex.initialized);
>> >      qemu_mutex_lock(&qemu_logfile_mutex);
>> > -    if (qemu_logfile) {
>> > -        if (qemu_logfile != stderr) {
>> > -            fclose(qemu_logfile);
>> > -        }
>> > -        qemu_logfile = NULL;
>> > +    logfile = qemu_logfile;
>> > +
>> > +    if (logfile) {
>> > +        atomic_rcu_set(&qemu_logfile, NULL);
>> > +        call_rcu(logfile, qemu_logfile_free, rcu);
>> >      }
>> >      qemu_mutex_unlock(&qemu_logfile_mutex);
>> >  }
>> >  diff --git a/include/exec/log.h b/include/exec/log.h
>> > index e2cfd436e6..9bd1e4aa20 100644
>> > --- a/include/exec/log.h
>> > +++ b/include/exec/log.h
>> > @@ -15,8 +15,15 @@
>> >   */
>> >  static inline void log_cpu_state(CPUState *cpu, int flags)
>> >  {
>> > +    QemuLogFile *logfile;
>> > +
>> >      if (qemu_log_enabled()) {
>> > -        cpu_dump_state(cpu, qemu_logfile, flags);
>> > +        rcu_read_lock();
>> > +        logfile = atomic_rcu_read(&qemu_logfile);
>> > +        if (logfile) {
>> > +            cpu_dump_state(cpu, logfile->fd, flags);
>> > +        }
>> > +        rcu_read_unlock();
>> >      }
>> >  }
>> >
>> > @@ -40,19 +47,37 @@ static inline void log_cpu_state_mask(int mask, 
>> > CPUState *cpu, int flags)
>> >  static inline void log_target_disas(CPUState *cpu, target_ulong start,
>> >                                      target_ulong len)
>> >  {
>> > -    target_disas(qemu_logfile, cpu, start, len);
>> > +    QemuLogFile *logfile;
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        target_disas(logfile->fd, cpu, start, len);
>> > +    }
>> > +    rcu_read_unlock();
>> >  }
>> >
>> >  static inline void log_disas(void *code, unsigned long size)
>> >  {
>> > -    disas(qemu_logfile, code, size);
>> > +    QemuLogFile *logfile;
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        disas(logfile->fd, code, size);
>> > +    }
>> > +    rcu_read_unlock();
>> >  }
>> >
>> >  #if defined(CONFIG_USER_ONLY)
>> >  /* page_dump() output to the log file: */
>> >  static inline void log_page_dump(void)
>> >  {
>> > -    page_dump(qemu_logfile);
>> > +    QemuLogFile *logfile;
>> > +    rcu_read_lock();
>> > +    logfile = atomic_rcu_read(&qemu_logfile);
>> > +    if (logfile) {
>> > +        page_dump(logfile->fd);
>> > +    }
>> > +    rcu_read_unlock();
>> >  }
>> >  #endif
>> >  #endif
>> > diff --git a/tcg/tcg.c b/tcg/tcg.c
>> > index 0511266d85..4f616ba38b 100644
>> > --- a/tcg/tcg.c
>> > +++ b/tcg/tcg.c
>> > @@ -2114,9 +2114,17 @@ static void tcg_dump_ops(TCGContext *s, bool 
>> > have_prefs)
>> >          }
>> >
>> >          if (have_prefs || op->life) {
>> > -            for (; col < 40; ++col) {
>> > -                putc(' ', qemu_logfile);
>> > +
>> > +            QemuLogFile *logfile;
>> > +
>> > +            rcu_read_lock();
>> > +            logfile = atomic_rcu_read(&qemu_logfile);
>> > +            if (logfile) {
>> > +                for (; col < 40; ++col) {
>> > +                    putc(' ', logfile->fd);
>> > +                }
>> >              }
>> > +            rcu_read_unlock();
>> >          }
>> >
>> >          if (op->life) {
>>
>>
>> --
>> Alex Bennée


--
Alex Bennée