On 2019-08-30 07:19, Maxim Dounin wrote:
Hello!
On Fri, Aug 23, 2019 at 11:00:41AM -0700, Rian Hunter wrote:
While browsing the source I noticed something that seemed wrong, even
though I haven't observed any buggy behavior or have reproduced this
bug
myself. In ngx_event.c there is a line:
if (ngx_shmtx_create(&ngx_accept_mutex, (ngx_shmtx_sh_t *)
shared,
cycle->lock_file.data)
!= NGX_OK)
{
return NGX_ERROR;
}
ngx_shmtx_create() is passed &ngx_accept_mutex, but this must be a
pointer to a shared memory region otherwise the sem_wait()/sem_post()
calls in ngx_shmtx.c will not function correctly. &ngx_accept_mutex is
a
pointer to BSS, ngx_accept_mutex_ptr is a pointer to shared memory. Is
this intentional?
Yes, this is intentional.
The first agument of the ngx_shmtx_create() function is a pointer
to the ngx_shmtx_t structure, which is not expected to be shared
between processes. Copy of the structure as created by fork() is
enough.
The POSIX sem_t member (called "sem") needs to reside in shared memory
if sem_wait()/sem_post() are to have an effect across processes. Memory
copied across fork is not sufficient.
Only ngx_shmtx_sh_t - which is used to store atomic variables with
lock and wait counter - needs to be shared. And it is shared, as
"(ngx_shmtx_sh_t *) shared" is used as the second argument.
Yes, the atomic modifications to the variable stored in that memory is
correct and will be shared across processes but the
sem_wait()/sem_post() calls
within ngx_shmtx.c have no effect since the "sem" member of ngx_shmtx_t
is not shared.
The end result will be that ngx_shmtx_wakeup() (amongst others) has no
effect.
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