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Groups > linux.kernel > #1622801 > unrolled thread
| Started by | Laurent Dufour <ldufour@linux.vnet.ibm.com> |
|---|---|
| First post | 2017-04-13 10:10 +0200 |
| Last post | 2017-04-13 11:00 +0200 |
| Articles | 3 — 2 participants |
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Re: [PATCH 2/6] locking: Introduce range reader/writer lock Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-04-13 10:10 +0200
Re: [PATCH 2/6] locking: Introduce range reader/writer lock Jan Kara <jack@suse.cz> - 2017-04-13 10:40 +0200
Re: [PATCH 2/6] locking: Introduce range reader/writer lock Laurent Dufour <ldufour@linux.vnet.ibm.com> - 2017-04-13 11:00 +0200
| From | Laurent Dufour <ldufour@linux.vnet.ibm.com> |
|---|---|
| Date | 2017-04-13 10:10 +0200 |
| Subject | Re: [PATCH 2/6] locking: Introduce range reader/writer lock |
| Message-ID | <tvHZf-89-3@gated-at.bofh.it> |
On 06/04/2017 18:50, Davidlohr Bueso wrote: > On Thu, 06 Apr 2017, Laurent Dufour wrote: > >> How is 'seqnum' wrapping handled here ? >> I'd rather see something like time_before() here, isn't it ? > > Its a 64bit counter, no overflows. I should have miss something, what prevents this 64bit counter to not overflow ? At some point of time, this counter could reach ~0UL and then 0UL, which is breaking this check.
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| From | Jan Kara <jack@suse.cz> |
|---|---|
| Date | 2017-04-13 10:40 +0200 |
| Message-ID | <tvIsh-kh-17@gated-at.bofh.it> |
| In reply to | #1622801 |
On Thu 13-04-17 10:07:48, Laurent Dufour wrote: > On 06/04/2017 18:50, Davidlohr Bueso wrote: > > On Thu, 06 Apr 2017, Laurent Dufour wrote: > > > >> How is 'seqnum' wrapping handled here ? > >> I'd rather see something like time_before() here, isn't it ? > > > > Its a 64bit counter, no overflows. > > I should have miss something, what prevents this 64bit counter to not > overflow ? > At some point of time, this counter could reach ~0UL and then 0UL, which > is breaking this check. Count for yourself how long would it take for the counter to overflow if we incremented it say every nanosecond? Honza -- Jan Kara <jack@suse.com> SUSE Labs, CR
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| From | Laurent Dufour <ldufour@linux.vnet.ibm.com> |
|---|---|
| Date | 2017-04-13 11:00 +0200 |
| Message-ID | <tvILD-sK-5@gated-at.bofh.it> |
| In reply to | #1622822 |
On 13/04/2017 10:38, Jan Kara wrote: > On Thu 13-04-17 10:07:48, Laurent Dufour wrote: >> On 06/04/2017 18:50, Davidlohr Bueso wrote: >>> On Thu, 06 Apr 2017, Laurent Dufour wrote: >>> >>>> How is 'seqnum' wrapping handled here ? >>>> I'd rather see something like time_before() here, isn't it ? >>> >>> Its a 64bit counter, no overflows. >> >> I should have miss something, what prevents this 64bit counter to not >> overflow ? >> At some point of time, this counter could reach ~0UL and then 0UL, which >> is breaking this check. > > Count for yourself how long would it take for the counter to overflow if we > incremented it say every nanosecond? I do agree that this will take time, but systems become larger and having 200 threads hitting a lock like the mmap_sem every nanoseconds, it will increment faster (still need about 3 years)... if there are 1000 threads, less than 1 year to overflow this counter... This is not for today, I agree.
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