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    SubjectRe: [PATCH] Use ERESTART_RESTARTBLOCK if poll() is interrupted by a signal (was: Re: [PATCH] Use ERESTARTNOHAND if poll() is interrupted by a signal)
    On 08/03, Chris Wright wrote:
    >
    > +long do_restart_poll(struct restart_block *restart_block)
    > +{
    > + struct pollfd __user *ufds = (struct pollfd __user*)restart_block->arg0;
    > + int nfds = restart_block->arg1;
    > + s64 timeout = ((s64)restart_block->arg3<<32) | (s64)restart_block->arg2;
    > + int ret;
    > +
    > + restart_block->fn = do_no_restart_syscall;

    (just in case, this is not strictly necessary)

    > + ret = do_sys_poll(ufds, nfds, &timeout);
    > + if (ret == -EINTR) {
    > + restart_block->fn = do_restart_poll;
    > + restart_block->arg2 = timeout & 0xFFFFFFFF;
    > + restart_block->arg3 = timeout >> 32;
    > + ret = -ERESTART_RESTARTBLOCK;
    > + }
    > + return ret;
    > +}
    > +
    > asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,
    > long timeout_msecs)
    > {
    > s64 timeout_jiffies;
    > + int ret;
    >
    > if (timeout_msecs > 0) {
    > #if HZ > 1000
    > @@ -754,7 +773,20 @@ asmlinkage long sys_poll(struct pollfd __user *ufds, unsigned int nfds,
    > timeout_jiffies = timeout_msecs;
    > }
    >
    > - return do_sys_poll(ufds, nfds, &timeout_jiffies);
    > + ret = do_sys_poll(ufds, nfds, &timeout_jiffies);
    > + if (ret == -EINTR) {
    > + if (timeout_msecs > 0) {

    This should be "if (timeout_msecs)", a negative timeout means
    "wait indefinitely", so we should restart as well.

    > + struct restart_block *restart_block;
    > + restart_block = &current_thread_info()->restart_block;
    > + restart_block->fn = do_restart_poll;
    > + restart_block->arg0 = (unsigned long)ufds;
    > + restart_block->arg1 = nfds;
    > + restart_block->arg2 = timeout_jiffies & 0xFFFFFFFF;
    > + restart_block->arg3 = timeout_jiffies >> 32;

    and, in that case, we should use "(u64)timeout_jiffies >> 32".


    Small nit: sys_poll() and do_restart_poll() are not "symmetrical", the latter
    doesn't check *timeout at all. Either way is correct, restart with zero timeout
    means "try again once but doesn't wait".

    There is a subtle (but harmless) difference though. Suppose that *timeout == 0
    (either because timeout_msecs == 0 or because timeout expiried) and we have a
    "false" signal_pending(). In that case sys_poll() returns EINTR, but do_restart_poll()
    returns 0.

    I was a bit surprized that sys_poll() return EINTR even if timeout expired, but
    preserved this behaviour in do_poll-return-eintr-when-signalled.patch.

    Perhaps it is better to just remove "if (timeout_msecs > 0)" check from sys_poll().
    Then we can modify do_poll() to return EINTR only when !*timeout, to eliminate
    unneeded (but correct) restart.

    What do you think?

    Oleg.

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      Last update: 2007-08-04 13:09    [W:0.029 / U:0.024 seconds]
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