When I researched synchronisation behavior and wrote the wiki page
"Threads", I found that RTL events behave like this:
- When the event is set, a single waiting thread is released (in FIFO
order), and the event is automatically immediately reset.
- The event can only be in a set or unset state; multiple sets do not
stack. Even if the event is set twice or ten times, a single WaitFor or
Reset will change it to unset. This is a likely cause for deadlocks.
- Because the event is automatically reset when a waiting thread is
released, calling ResetEvent is generally unnecessary.
For set/reset counting we'd need a semaphore instead of an RTL event,
but I don't think we have an actively supported semaphore implementation
at this time.
Caveat: The above was true for FPC 3.0.4 on Windows and Linux. 3.2.0 or
trunk may have changed something, and I can't vouch for other platforms.
~Kirinn
On 8/22/20 1:44 AM, Martin via fpc-devel wrote:
I am tracing a race condition in lazCollections TLazThreadedQueue
on win-64
What is supposed to happen in the following case:
2 or more threads are waiting on the some event
RTLeventWaitFor(UniqueEvent)
1 thread sets it
RTLeventSetEvent(UniqueEvent)
I expect that wakes up exactly ONE thread (never mind wich)?
Or ALL?
If no one resets the event (NO call to RTLeventResetEvent), then that
does not change anything? It is still just the one thread that got
woken up?
Thanks for any feedback
Martin
p.s.
Background
TLazThreadedQueue.PopItem works like this
EnterCritical
If item avail then return item
ExitCritical
// race condition between those lines.
RTLeventWaitFor(itemAdded)
EnterCritical
If item avail then return
ExitCritical
If 2 or more reading (popping) threads are both at the indicated
location,
and if during this time 2 or more items are added
then there is a problem (I believe)
adding (pushing) an item does
RTLeventSet(itemAdded)
adding 2 items (with no pop in between) calls Set twice
=>
https://lazarus-ccr.sourceforge.io/docs/rtl/system/rtleventsetevent.html
It is unclear if the 2nd RTLeventSet has any effect ?
According to my tests:
- I "sleep()" 2 listeners at the critical line, so they both will
WaitFor after the sleep
- while they sleep I add 2 items (2 calls to RTLeventSet, no one
listening yet)
- Both start RTLeventWaitFor
=> only one listener wakes up.
Is that the expected behaviour?
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