[Bug 1886155] Re: error: argument 2 of ‘__atomic_load’ discards ‘const’ qualifier

[Jonathan Wakely]

Which means that given an argument of type T * const this defines a
local variable that is also T * const, and then tries to store the
result of the atomic load into that const variable:


```
#define atomic_rcu_read(ptr)                          \
    ({                                                \
    typeof_strip_qual(*ptr) _val;                     \
    atomic_rcu_read__nocheck(ptr, &_val);             \
    _val;                                             \
    })
```

GCC 11 correctly diagnoses that write to a const variable.

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https://bugs.launchpad.net/bugs/1886155

Title:
  error: argument 2 of ‘__atomic_load’ discards ‘const’ qualifier

Status in QEMU:
  New

Bug description:
  GCC 11 reports the following errors:

  [  125s] In file included from 
/home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/seqlock.h:17,
  [  125s]                  from 
/home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/qht.h:10,
  [  125s]                  from 
/home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:69:
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c: In function 
'qht_do_lookup':
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:153:5: 
error: argument 2 of '__atomic_load' discards 'const' qualifier 
[-Werror=incompatible-pointer-types]
  [  125s]   153 |     __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
  [  125s]       |     ^~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:161:5: 
note: in expansion of macro 'atomic_rcu_read__nocheck'
  [  125s]   161 |     atomic_rcu_read__nocheck(ptr, &_val);             \
  [  125s]       |     ^~~~~~~~~~~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:499:27: note: in 
expansion of macro 'atomic_rcu_read'
  [  125s]   499 |                 void *p = atomic_rcu_read(&b->pointers[i]);
  [  125s]       |                           ^~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:153:5: 
error: argument 2 of '__atomic_load' discards 'const' qualifier 
[-Werror=incompatible-pointer-types]
  [  125s]   153 |     __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
  [  125s]       |     ^~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:161:5: 
note: in expansion of macro 'atomic_rcu_read__nocheck'
  [  125s]   161 |     atomic_rcu_read__nocheck(ptr, &_val);             \
  [  125s]       |     ^~~~~~~~~~~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:506:13: note: in 
expansion of macro 'atomic_rcu_read'
  [  125s]   506 |         b = atomic_rcu_read(&b->next);
  [  125s]       |             ^~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c: In function 
'qht_lookup_custom':
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:153:5: 
error: argument 2 of '__atomic_load' discards 'const' qualifier 
[-Werror=incompatible-pointer-types]
  [  125s]   153 |     __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
  [  125s]       |     ^~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:161:5: 
note: in expansion of macro 'atomic_rcu_read__nocheck'
  [  125s]   161 |     atomic_rcu_read__nocheck(ptr, &_val);             \
  [  125s]       |     ^~~~~~~~~~~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:534:11: note: in 
expansion of macro 'atomic_rcu_read'
  [  125s]   534 |     map = atomic_rcu_read(&ht->map);
  [  125s]       |           ^~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c: In function 
'qht_statistics_init':
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:153:5: 
error: argument 2 of '__atomic_load' discards 'const' qualifier 
[-Werror=incompatible-pointer-types]
  [  125s]   153 |     __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
  [  125s]       |     ^~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:161:5: 
note: in expansion of macro 'atomic_rcu_read__nocheck'
  [  125s]   161 |     atomic_rcu_read__nocheck(ptr, &_val);             \
  [  125s]       |     ^~~~~~~~~~~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:907:11: note: in 
expansion of macro 'atomic_rcu_read'
  [  125s]   907 |     map = atomic_rcu_read(&ht->map);
  [  125s]       |           ^~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:153:5: 
error: argument 2 of '__atomic_load' discards 'const' qualifier 
[-Werror=incompatible-pointer-types]
  [  125s]   153 |     __atomic_load(ptr, valptr, __ATOMIC_RELAXED);       \
  [  125s]       |     ^~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/include/qemu/atomic.h:161:5: 
note: in expansion of macro 'atomic_rcu_read__nocheck'
  [  125s]   161 |     atomic_rcu_read__nocheck(ptr, &_val);             \
  [  125s]       |     ^~~~~~~~~~~~~~~~~~~~~~~~
  [  125s] /home/abuild/rpmbuild/BUILD/qemu-5.0.0/util/qht.c:941:21: note: in 
expansion of macro 'atomic_rcu_read'
  [  125s]   941 |                 b = atomic_rcu_read(&b->next);
  [  125s]       |                     ^~~~~~~~~~~~~~~

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