allocation within critical sections

[Andy Wingo]

Hi,

FYI: Allocating memory can run finalizers, which can run arbitrary code.
So if you allocate memory from within a critical section, i.e. while a
mutex is held, you need to be really sure that there's no way a
finalizer would try to grab the lock you are already holding.

In some cases it's best to simply avoid allocation while holding a lock.

Andy, who got the following backtrace, with wip-threads-and-fork:

#0  0x00007f5daf443cec in __lll_lock_wait () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#1  0x00007f5daf43f339 in _L_lock_926 () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#2  0x00007f5daf43f15b in pthread_mutex_lock () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#3  0x00007f5dafba8563 in finalize_port (ptr=0x3dac5a0, data=<optimized out>) 
at ports.c:575
#4  finalize_port (ptr=0x3dac5a0, data=<optimized out>) at ports.c:551
#5  0x00007f5daee5851e in GC_invoke_finalizers () at finalize.c:872
#6  0x00007f5daee5876e in GC_notify_or_invoke_finalizers () at finalize.c:951
#7  0x00007f5daee5ab4d in GC_generic_malloc_many (lb=16, k=1, 
result=0x7f5daf080d30) at mallocx.c:290
#8  0x00007f5daee640a1 in GC_malloc (bytes=16) at thread_local_alloc.c:177
#9  0x00007f5dafba690e in scm_cell (cdr=772, car=2052) at ../libguile/gc.h:204
#10 scm_cons (x=<optimized out>, y=0x304) at pairs.c:77
#11 0x00007f5dafb8cc6e in scm_make_list (n=<optimized out>, init=0x804) at 
list.c:120
#12 0x00007f5dafb82c39 in finalize_guarded (ptr=<optimized out>, 
finalizer_data=<optimized out>) at guardians.c:124
#13 0x00007f5daee5851e in GC_invoke_finalizers () at finalize.c:872
#14 0x00007f5daee5876e in GC_notify_or_invoke_finalizers () at finalize.c:951
#15 0x00007f5daee5ab4d in GC_generic_malloc_many (lb=16, k=1, 
result=0x7f5daf080d30) at mallocx.c:290
#16 0x00007f5daee640a1 in GC_malloc (bytes=16) at thread_local_alloc.c:177
#17 0x00007f5dafba690e in scm_cell (cdr=65961984, car=36468752) at 
../libguile/gc.h:204
#18 scm_cons (x=<optimized out>, y=0x3ee8000) at pairs.c:77
#19 0x00007f5dafbf5b6f in lock_all_weak_tables (unused=<optimized out>) at 
weak-table.c:762
#20 0x00007f5dafbf8c97 in before_fork () at posix.c:1295
#21 scm_fork () at posix.c:1328

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