Re: Race condition in storeClose()

On 22 Nov 2000, at 23:36, Adrian Chadd <adrian@creative.net.au> wrote:

> If there's a problem with an client FD, it will destroy the connection.
> Eventually it hits storeUnregister(), but it doesn't check if there are
> pending disk IOs (and close them if they DO exist.)
>
> I think this is a bad thing, and could be causing our problems with
> random memory trashings, since a pending read would complete, and
> you'd overwrite the memory if you're not careful.

 It is causing the problem. To chase it I flagged freed mempool
 objects as read-only vm-pages and watched who's writing them. Got
 consistently crashes from within aioread copying back to user buffer.

> So, ideas here? Should the FS handle killing pending disk IOs on a
> storeIOState on a storeClose() / storeUnlink() ? Or should we
> implement a storeCancel() call to cancel an IO, which is called from
> storeUnregister() .. ?

 logically, when we write()+close(), we expect data to reach disks,
 but when we close() before read() completes, we expect read to be
 canceled. So, I think storeClose should do. It just needs to make
 difference between read or writes.

> People running squid-HEAD / squid-2.4 and aufs, and can replicate
> the crashes, please try this patch just to try:
>
> if (storeAufsSomethingPending(sio)) {
> aiostate->flags.close_request = 1;
> - return;
> + /* return; */
> }
> storeAufsIOCallback(sio, DISK_OK);

 Other option is to call aioClose() before return, or just aioCancel().
 Current state simply asks aufs to close the fd after its pending op
 completes. With canceled reads this isn't what we want.

 Just one issue with this: do we actually need to allow pending IO to
 complete? Perhaps we don't want to cancel aiowrite but want to append
 a close command into the aio queue.
 If we simply cancel pending io above, could we possibly have partially
 written objects in store?

 Yet we have to cancel pending read instantly, I believe.

------------------------------------
 Andres Kroonmaa <andre@online.ee>
 Delfi Online
 Tel: 6501 731, Fax: 6501 708
 Pärnu mnt. 158, Tallinn,
 11317 Estonia
Received on Wed Nov 22 2000 - 11:22:43 MST