Date: Fri, 9 Sep 2005 19:55:34 -0700
From: Nishanth Aravamudan <nacc@us.ibm.com>
To: Andrew Morton <akpm@osdl.org>
Cc: dwmw2@infradead.org, bunk@stusta.de, johnstul@us.ibm.com,
       drepper@redhat.com, Franz.Fischer@goyellow.de,
       linux-kernel@vger.kernel.org
Subject: Re: [UPDATE PATCH][Bug 5132] fix sys_poll() large timeout handling
Message-ID: <20050910025534.GE24225@us.ibm.com>
References: <20050831200109.GB3017@us.ibm.com> <20050906212514.GB3038@us.ibm.com> <20050910003525.GC24225@us.ibm.com> <20050909181658.221eb6f9.akpm@osdl.org> <20050910022330.GD24225@us.ibm.com> <20050909193621.5d578583.akpm@osdl.org>
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On 09.09.2005 [19:36:21 -0700], Andrew Morton wrote:
> Nishanth Aravamudan <nacc@us.ibm.com> wrote:
> >
> > +	/*
> >  +	 * We compare HZ with 1000 to work out which side of the
> >  +	 * expression needs conversion.  Because we want to avoid
> >  +	 * converting any value to a numerically higher value, which
> >  +	 * could overflow.
> >  +	 */
> >  +#if HZ > 1000
> >  +	overflow = timeout_msecs >= jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT);
> >  +#else
> >  +	overflow = msecs_to_jiffies(timeout_msecs) >= MAX_SCHEDULE_TIMEOUT;
> >  +#endif
> >  +
> >  +	/*
> >  +	 * If we would overflow in the conversion or a negative timeout
> >  +	 * is requested, sleep indefinitely.
> >  +	 */
> >  +	if (overflow || timeout_msecs < 0)
> >  +		timeout_jiffies = MAX_SCHEDULE_TIMEOUT;
> 
> Do we need to test (timeout_msecs < 0) here?  If we make timeout_msecs
> unsigned long then I think `overflow' will always be correct.

Even though poll is explicitly allowed to take negative values, as per
my man-page:

"#include <sys/poll.h>

int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

...

A negative value means infinite timeout."

Would we have a local variable to store timeout_msecs as well? Or do we
want to make a userspace-visible change like this? I don't have a
preference, I just want to make sure I understand.

Thanks,
Nish
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