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href="/lkml/2025/3/13/617">"tip-bot2 for Thomas Gleixner"</a></li></ul></li></ul></li></ul></li></ul><div class="threadlist">Patch in this message</div><ul class="threadlist"><li><a href="/lkml/diff/2025/3/13/623/1">Get diff 1</a></li></ul></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerl.gif" width="32" height="32" alt="/" /></td><td class="c" rowspan="2" valign="top" style="padding-top: 1em"><table><tr><td><table><tr><td class="lp">Date</td><td class="rp" itemprop="datePublished">Thu, 13 Mar 2025 11:31:26 -0000</td></tr><tr><td class="lp">From</td><td class="rp" itemprop="author">"tip-bot2 for Eric Dumazet" <></td></tr><tr><td class="lp">Subject</td><td class="rp" itemprop="name">[tip: timers/core] posix-timers: Make signal_struct:: Next_posix_timer_id an atomic_t</td></tr></table></td><td></td></tr></table><pre itemprop="articleBody">The following commit has been merged into the timers/core branch of tip:<br /><br />Commit-ID: feb864ee99a2d8a22800342388401f3a3b90d42b<br />Gitweb: <a href="https://git.kernel.org/tip/feb864ee99a2d8a22800342388401f3a3b90d42b">https://git.kernel.org/tip/feb864ee99a2d8a22800342388401f3a3b90d42b</a><br />Author: Eric Dumazet <edumazet@google.com><br />AuthorDate: Sat, 08 Mar 2025 17:48:36 +01:00<br />Committer: Thomas Gleixner <tglx@linutronix.de><br />CommitterDate: Thu, 13 Mar 2025 12:07:17 +01:00<br /><br />posix-timers: Make signal_struct:: Next_posix_timer_id an atomic_t<br /><br />The global hash_lock protecting the posix timer hash table can be heavily<br />contended especially when there is an extensive linear search for a timer<br />ID.<br /><br />Timer IDs are handed out by monotonically increasing next_posix_timer_id<br />and then validating that there is no timer with the same ID in the hash<br />table. Both operations happen with the global hash lock held.<br /><br />To reduce the hash lock contention the hash will be reworked to a scaled<br />hash with per bucket locks, which requires to handle the ID counter<br />lockless.<br /><br />Prepare for this by making next_posix_timer_id an atomic_t, which can be<br />used lockless with atomic_inc_return().<br /><br />[ tglx: Adopted from Eric's series, massaged change log and simplified it ]<br /><br />Signed-off-by: Eric Dumazet <edumazet@google.com><br />Signed-off-by: Thomas Gleixner <tglx@linutronix.de><br />Reviewed-by: Frederic Weisbecker <frederic@kernel.org><br />Acked-by: Frederic Weisbecker <frederic@kernel.org><br />Link: <a href="https://lore.kernel.org/all/20250219125522.2535263-2-edumazet@google.com">https://lore.kernel.org/all/20250219125522.2535263-2-edumazet@google.com</a><br />Link: <a href="https://lore.kernel.org/all/20250308155624.151545978@linutronix.de">https://lore.kernel.org/all/20250308155624.151545978@linutronix.de</a><br /><br /><br />---<br /> include/linux/sched/signal.h | 2 +-<br /> kernel/time/posix-timers.c | 14 +++++---------<br /> 2 files changed, 6 insertions(+), 10 deletions(-)<br /><br />diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h<br />index d5d03d9..72649d7 100644<br />--- a/include/linux/sched/signal.h<br />+++ b/include/linux/sched/signal.h<br />@@ -136,7 +136,7 @@ struct signal_struct {<br /> #ifdef CONFIG_POSIX_TIMERS<br /> <br /> /* POSIX.1b Interval Timers */<br />- unsigned int next_posix_timer_id;<br />+ atomic_t next_posix_timer_id;<br /> struct hlist_head posix_timers;<br /> struct hlist_head ignored_posix_timers;<br /> <br />diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c<br />index 991d12a..f9a70c1 100644<br />--- a/kernel/time/posix-timers.c<br />+++ b/kernel/time/posix-timers.c<br />@@ -119,21 +119,17 @@ static bool posix_timer_hashed(struct hlist_head *head, struct signal_struct *si<br /> static int posix_timer_add(struct k_itimer *timer)<br /> {<br /> struct signal_struct *sig = current->signal;<br />- struct hlist_head *head;<br />- unsigned int cnt, id;<br /> <br /> /*<br /> * FIXME: Replace this by a per signal struct xarray once there is<br /> * a plan to handle the resulting CRIU regression gracefully.<br /> */<br />- for (cnt = 0; cnt <= INT_MAX; cnt++) {<br />- spin_lock(&hash_lock);<br />- id = sig->next_posix_timer_id;<br />-<br />- /* Write the next ID back. Clamp it to the positive space */<br />- sig->next_posix_timer_id = (id + 1) & INT_MAX;<br />+ for (unsigned int cnt = 0; cnt <= INT_MAX; cnt++) {<br />+ /* Get the next timer ID and clamp it to positive space */<br />+ unsigned int id = atomic_fetch_inc(&sig->next_posix_timer_id) & INT_MAX;<br />+ struct hlist_head *head = &posix_timers_hashtable[hash(sig, id)];<br /> <br />- head = &posix_timers_hashtable[hash(sig, id)];<br />+ spin_lock(&hash_lock);<br /> if (!posix_timer_hashed(head, sig, id)) {<br /> /*<br /> * Set the timer ID and the signal pointer to make<br /></pre></td><td width="32" rowspan="2" class="c" valign="top"><img src="/images/icornerr.gif" width="32" height="32" alt="\" /></td></tr><tr><td align="right" valign="bottom"> 聽 </td></tr><tr><td align="right" valign="bottom">聽</td><td class="c" valign="bottom" style="padding-bottom: 0px"><img src="/images/bcornerl.gif" width="32" height="32" alt="\" /></td><td class="c">聽</td><td class="c" valign="bottom" style="padding-bottom: 0px"><img src="/images/bcornerr.gif" width="32" height="32" alt="/" /></td></tr><tr><td align="right" valign="top" colspan="2"> 聽 </td><td class="lm">Last update: 2025-03-13 12:33 聽聽 [W:0.383 / U:2.551 seconds]<br />漏2003-2020 <a href="http://blog.jasper.es/"><span itemprop="editor">Jasper Spaans</span></a>|hosted at <a href="https://www.digitalocean.com/?refcode=9a8e99d24cf9">Digital Ocean</a> and my Meterkast|<a href="http://blog.jasper.es/categories.html#lkml-ref">Read the blog</a></td><td>聽</td></tr></table><script language="javascript" src="/js/styleswitcher.js" type="text/javascript"></script></body></html>