Re: [PATCH] newlib: Unlock the mutex while calling atexit()'ed functions

Hello Takashi Yano,

I have some questions to the change.

----- Am 18. Nov 2025 um 14:31 schrieb Takashi Yano [email protected]:

> The atexit()'ed function may deadlock if it waits for another thread
> which calls atexit() in the current __call_atexit.c code. This patch
> unlock __atexit_recursive_mutex while calling atexit()'ed functions
> to avoid the deadlock mentioned above. glibc and Darwin do the same,
> so it sounds reasonable.
> 
> Addresses: https://cygwin.com/pipermail/cygwin/2025-October/258930.html
> Reported-by: Tomohiro Kashiwada <[email protected]>
> Reviewed-by:
> Signed-off-by: Takashi Yano <[email protected]>
> ---
> newlib/libc/stdlib/__call_atexit.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/newlib/libc/stdlib/__call_atexit.c
> b/newlib/libc/stdlib/__call_atexit.c
> index 710440389..44f1f6acc 100644
> --- a/newlib/libc/stdlib/__call_atexit.c
> +++ b/newlib/libc/stdlib/__call_atexit.c
> @@ -114,6 +114,11 @@ __call_exitprocs (int code, void *d)
> 
> 	  ind = p->_ind;
> 
> +#ifndef __SINGLE_THREAD__
> +	  /* Unlock __atexit_recursive_mutex; otherwise, the function fn() may
> +	     deadlock if it waits for another thread which calls atexit(). */
> +	  __lock_release_recursive(__atexit_recursive_mutex);
> +#endif

Here, a potentially shared structure is used through the args pointer. If some other thread calls exit() or atexit() concurrently, then this could result in a use of altered or freed memory. The data of the structure referenced by args should be first copied to local variables. Also I think that the deallocation should be done before we release the lock. We probably also have to restart the process after each handler call unconditionally.


> 	  /* Call the function.  */
> 	  if (!args || (args->_fntypes & i) == 0)
> 	    fn ();
> @@ -121,6 +126,9 @@ __call_exitprocs (int code, void *d)
> 	    (*((void (*)(int, void *)) fn))(code, args->_fnargs[n]);
> 	  else
> 	    (*((void (*)(void *)) fn))(args->_fnargs[n]);
> +#ifndef __SINGLE_THREAD__
> +	  __lock_acquire_recursive(__atexit_recursive_mutex);
> +#endif
> 
> 	  /* The function we called call atexit and registered another
> 	     function (or functions).  Call these new functions before
> --
> 2.51.0

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