Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030410AbVIJBU5 (ORCPT ); Fri, 9 Sep 2005 21:20:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030411AbVIJBU5 (ORCPT ); Fri, 9 Sep 2005 21:20:57 -0400 Received: from smtp.osdl.org ([65.172.181.4]:41962 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1030410AbVIJBU5 (ORCPT ); Fri, 9 Sep 2005 21:20:57 -0400 Date: Fri, 9 Sep 2005 18:16:58 -0700 From: Andrew Morton To: Nishanth Aravamudan 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: <20050909181658.221eb6f9.akpm@osdl.org> In-Reply-To: <20050910003525.GC24225@us.ibm.com> References: <20050831200109.GB3017@us.ibm.com> <20050906212514.GB3038@us.ibm.com> <20050910003525.GC24225@us.ibm.com> X-Mailer: Sylpheed version 1.0.4 (GTK+ 1.2.10; i386-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2754 Lines: 78 Nishanth Aravamudan wrote: > > Description: The current sys_poll() implementation does not seem to > handle large timeouts correctly. Any value in milliseconds (@timeout) > which exceeds the maximum representable jiffy value > (MAX_SCHEDULE_TIMEOUT) should result in a MAX_SCHEDULE_TIMEOUT > schedule_timeout() request. To achieve this, convert @timeout to jiffies > first, then compare to MAX_SCHEDULE_TIMEOUT. The above doesn't describe the bug very well. > Signed-off-by: Nishanth Aravamudan > > --- > > fs/select.c | 17 ++++++++++------- > 1 file changed, 10 insertions(+), 7 deletions(-) > > diff -urpN 2.6.13/fs/select.c 2.6.13-dev/fs/select.c > --- 2.6.13/fs/select.c 2005-08-28 17:46:14.000000000 -0700 > +++ 2.6.13-dev/fs/select.c 2005-09-09 17:22:30.000000000 -0700 > @@ -469,13 +469,16 @@ asmlinkage long sys_poll(struct pollfd _ > if (nfds > current->files->max_fdset && nfds > OPEN_MAX) > return -EINVAL; > > - if (timeout) { > - /* Careful about overflow in the intermediate values */ > - if ((unsigned long) timeout < MAX_SCHEDULE_TIMEOUT / HZ) This is the problem to which you're referring, yes? We're comparing milliseconds with jiffies/HZ, yes? > - timeout = (unsigned long)(timeout*HZ+999)/1000+1; > - else /* Negative or overflow */ > - timeout = MAX_SCHEDULE_TIMEOUT; > - } > + if (timeout > 0) > + /* > + * Convert the value from msecs to jiffies - if overflow > + * occurs we get a negative value, which gets handled by > + * the next block > + */ > + timeout = msecs_to_jiffies(timeout) + 1; > + if (timeout < 0) /* Negative requests result in infinite timeouts */ > + timeout = MAX_SCHEDULE_TIMEOUT; > + /* 0 case falls through */ I don't particularly like the idea of relying on msecs_to_jiffies(too much) returning a negative value. Why can't we do int too_much; /* * 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 too_much = timeout > jiffies_to_msecs(MAX_SCHEDULE_TIMEOUT); #else too_much = msecs_to_jiffies(timeout) > MAX_SCHEDULE_TIMEOUT; #endif if (too_much) timeout = MAX_SCHEDULE_TIMEOUT; And while we're there, let's stop using the same variable for two different units - it's horrid. How about we nuke `timeout' and create timeout_msecs and timeout_jiffies to show what units they're in? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/