synchronization in ltt-usertrace

Hello,

I'm currently walking through the usertrace code, and am confused by a
couple of code sections of the "fast" tracer.

My question is the use of a semaphore in ltt_reserve_slot function in
ltt-usertrae/ltt/ltt-usertrace-fast.h. I don't fully understand the
entire code yet, but I'm thinking that it's not signal-safe in the
following condition. I would appreciate if anyone could tell me
whether my understanding is correct or not.

1. Let's say the writer has just called reserve_slot and the
reservation starts at a new subbuffer.

2. Assuming the subbuffer is not corrupted, the writer will call
sem_wait.

3. The write successfully returns from sem_wait, but is preempted by a
signal after the signals being re-enabled, but before the cmpxchg.

4. The handler for the signal could call reserve_slot, find the new
slot starts at the beginning of the next subbuffer, because the writer
does not yet update the value of offset field.

So, if the above steps happened, sem_wait would be called twice, even
though the writer and its signal handler would eventually use the same
subbuffer. If my understanding is correct, the semaphore is
initialized to the number of the subbuffers. If it's true, it wouldn't
make sense to call sem_wait multiple times for a single subbuffer,
right?

BTW, in the same function (reserve_slot in ltt-usertrace-fast.h), I
don't yet understand why sem_post is called when the next subbuffer is
corrupted, which might explain why I don't understand the call to
sem_wait.

Thanks very much in advance,

Naoya Maruyama