Re: [libevl][PATCH] lib/mutex: init recursion count after trylock via the core

[Philippe Gerum <rpm@xenomai.org>]

Philippe Gerum <rpm@xenomai.org> writes:

> Luc Montier <luc.montier@etu.unistra.fr> writes:
>
>> evl_trylock_mutex() takes the slow path to the core when try_lock()
>> returns -ENODATA, e.g. for a thread running in-band. Unlike
>> evl_timedlock_mutex(), it does not initialize gate.nesting once
>> EVL_MONIOC_TRYENTER succeeds.
>>
>> For a recursive mutex, nesting then stays at 0 after the first lock
>> instead of 1. After N successful lock calls by the owner, nesting is
>> N - 1, so the (N - 1)th unlock releases the mutex and the last one
>> fails with -EPERM.
>>
>> Set nesting to 1 after a successful TRYENTER, as evl_timedlock_mutex()
>> does after EVL_MONIOC_ENTER.
>>
>> Signed-off-by: Luc Montier <luc.montier@etu.unistra.fr>
>> ---
>> Hello,
>>
>> This patch is against libevl r50-5 (e7dc1af, Jan 2025), the version
>> that matches my kernel (6.6.74, EVL ABI level 37, Raspberry Pi 5).
>> I could not test it on master, which requires ABI 47.
>>
>> Reproducer: a recursive mutex, a thread running in-band when it calls
>> evl_trylock_mutex() three times, then three evl_unlock_mutex() calls.
>> The last unlock fails with -EPERM, and gate.nesting reads 0, 1, 2 after
>> the locks. With the patch it reads 1, 2, 3 and all unlocks succeed.
>>
>> On master, this code now seems to live in lib/intrinsics/gate.c. Is
>> this already fixed there, and where would the fix belong?
>>
>> I just want to if it is an issue from libevl or a misuse on my part.
>>
>> Best regards,
>> Luc Montier
>>
>>
>>  lib/mutex.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/mutex.c b/lib/mutex.c
>> index 276e77f..832268f 100644
>> --- a/lib/mutex.c
>> +++ b/lib/mutex.c
>> @@ -332,6 +332,7 @@ int evl_lock_mutex(struct evl_mutex *mutex)
>>  
>>  int evl_trylock_mutex(struct evl_mutex *mutex)
>>  {
>> +	struct evl_monitor_state *gst;
>>  	int ret;
>>  
>>  	ret = try_lock(mutex);
>> @@ -341,7 +342,12 @@ int evl_trylock_mutex(struct evl_mutex *mutex)
>>  	do
>>  		ret = oob_ioctl(mutex->u.active.efd, EVL_MONIOC_TRYENTER);
>>  	while (ret && errno == EINTR);
>>
>> +	if (ret == 0) {
>> +		gst = mutex->u.active.state;
>> +		gst->u.gate.nesting = 1;
>> +	}
>> +
>>  	return ret ? -errno : 0;
>>  }
>
> Thanks for looking into this. I don't know about libevl r50{-5}, since
> we are not adding any patch sequence to libevl releases. This said, this
> issue was already fixed by commit 189e496e (ABI 44).
>

Note: this is a kernel-side commit in branch rebase/v6.12.y-cip-evl
available from all kernel releases we maintain as well.

-- 
Philippe.