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Unlock OS thread after netns is restored

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The current ns package code is very careful about not leaving the calling
thread with the overridden namespace set, for example when origns.Set() fails.
This is achieved by starting a new green thread, locking its OS thread, and
never unlocking it. Which makes golang runtime to scrap the OS thread backing
the green thread after the go routine exits.

While this works, it's probably not as optimal: stopping and starting a new OS
thread is expensive and may be avoided if we unlock the thread after resetting
network namespace to the original. On the other hand, if resetting fails, it's
better to leave the thread locked and die.

While it won't work in all cases, we can still make an attempt to reuse the OS
thread when resetting the namespace succeeds. This can be achieved by unlocking
the thread conditionally to the namespace reset success.

Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com>
This commit is contained in:
Ihar Hrachyshka 2020-02-20 17:08:50 -05:00
parent 32fc3ee9d3
commit 112288ecb2
1 changed files with 14 additions and 1 deletions
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15
pkg/ns/ns_linux.go
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@ -178,7 +178,16 @@ func (ns *netNS) Do(toRun func(NetNS) error) error {
if err = ns.Set(); err != nil {
return fmt.Errorf("error switching to ns %v: %v", ns.file.Name(), err)
}
defer threadNS.Set() // switch back
defer func() {
err := threadNS.Set() // switch back
if err == nil {
// Unlock the current thread only when we successfully switched back
// to the original namespace; otherwise leave the thread locked which
// will force the runtime to scrap the current thread, that is maybe
// not as optimal but at least always safe to do.
runtime.UnlockOSThread()
}
}()
return toRun(hostNS)
}
@ -193,6 +202,10 @@ func (ns *netNS) Do(toRun func(NetNS) error) error {
var wg sync.WaitGroup
wg.Add(1)
// Start the callback in a new green thread so that if we later fail
// to switch the namespace back to the original one, we can safely
// leave the thread locked to die without a risk of the current thread
// left lingering with incorrect namespace.
var innerError error
go func() {
defer wg.Done()