[PATCH][Bug 5132] fix sys_poll() large timeout handling

From: Nishanth Aravamudan (nacc_at_us.ibm.com)
Date: 08/31/05

  • Next message: Patrick Mochel: "Re: [linux-pm] Re: PowerOP Take 2 0/3 Intro" 
    Date:	Wed, 31 Aug 2005 13:01:09 -0700
To: akpm@osdl.org

    Sorry everybody, forgot the most important Cc: :)

    -Nish

    Hi Andrew,

    In looking at Bug 5132 and sys_poll(), I think there is a flaw in the
    current code.

    The @timeout parameter to sys_poll() is in milliseconds but we compare
    it to (MAX_SCHEDULE_TIMEOUT / HZ), which is jiffies/jiffies-per-sec or
    seconds. That seems blatantly broken. Also, I think we are better served
    by converting to jiffies first then comparing, as opposed to converting
    our maximum to milliseconds (or seconds, incorrectly) and comparing.

    Comments, suggestions for improvement?

    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() call. To achieve this, convert @timeout to jiffies
    first, then compare to MAX_SCHEDULE_TIMEOUT.

    Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com> 

    ---
 fs/select.c |   14 +++++++++-----
 1 files changed, 9 insertions(+), 5 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	2005-08-31 12:43:52.000000000 -0700
@@ -470,12 +470,16 @@ asmlinkage long sys_poll(struct pollfd _
 		return -EINVAL;
 
 	if (timeout) {
-		/* Careful about overflow in the intermediate values */
-		if ((unsigned long) timeout < MAX_SCHEDULE_TIMEOUT / HZ)
-			timeout = (unsigned long)(timeout*HZ+999)/1000+1;
-		else /* Negative or overflow */
-			timeout = MAX_SCHEDULE_TIMEOUT;
+		/*
+		 * 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 */
 
 	poll_initwait(&table);
 
-
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