Re: [PATCH] fix atexit when it is called from an atexit handler

[Jens Gustedt <jens.gustedt@inria.fr>]

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Am Donnerstag, den 23.07.2015, 07:54 +0200 schrieb Jens Gustedt:
> I'd think this could be done even simpler by running the handlers in
> batches. ...

I checked, the strict ordering constraints for the handlers don't
allow for such a strategy, so forget it.

Looking at the code, I am a bit uncomfortable with the idea of a
potentially unlimited number of calloc calls during exit. I think it
would be good if we could restrict the maximal number of successful
calls to atexit during exit to 32. A calloc-free strategy could be to
save head to a tmp a the beginning of processing and to provide a
`struct fl` table on the stack of __funcs_on_exit.

> A similar improvement could be done for at_quick_exit. There I think
> it makes not much sense accepting new handlers during processing. So
> the strategy could just be
> 
>   (i) Take the lock and save count in a tmp
>   (ii) set count to 32
>   (iii) unlock
>   (iv) do the processing
> 
> But looking at it, I think that one can even be more improved. We
> could just use count as atomic, and so we wouldn't need a lock.
> I can prepare a patch for that, if you like.

Since this is fairly simple, I just did it. Please find a new version
attached. (too much changes, a patch makes no sense.)  This compiles,
but I have no code to test this.

Jens

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#include <stdlib.h>
#include "atomic.h"
#include "libc.h"

#define COUNT 32

static void (*funcs[COUNT])(void);
static volatile int count;

void __funcs_on_quick_exit()
{
	void (*func)(void);
	/* No need for atomics here, we are alone. */
	int total = count;
	/* The counter might have been overrun at some time. */
	if (total > 32) total = 32;
 	else count = 32;
	while (total > 0) {
		func = funcs[--total];
		func();
	}
}

int at_quick_exit(void (*func)(void))
{
	int pos = a_fetch_add(&count, 1);
	if (pos >= 32) {
		if (pos > INT_MAX/2) pos = 32;
		return -1;
        }
	funcs[pos] = func;
	/* make sure that the effect is visible to everybody */
	a_barrier();
	return 0;
}

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